'Ballistic blocks' shoot from Hawaii volcano, may mark start of violent eruptions

VOLCANO, Hawaii, May 16 (Reuters) - "Ballistic blocks" the size of microwave ovens shot from Hawaii's Kilauea volcano on Wednesday in what may be the start of explosive eruptions that could spew huge ash plumes and hurl smaller rocks for miles (km), the U.S. Geological Survey said.

Such eruptions, last seen nearly a century ago, have been a looming threat since Kilauea, one of the world's most active volcanoes, erupted nearly two weeks ago.

Explosions in Kilauea's crater sparked an aviation red alert due to risks the ash plume could blow into aircraft routes and damage jet engines.

More explosions are expected and may be more powerful, the USGS warned. These steam-driven blasts could send a 20,000-foot (6,100-meter) ash plume out of the crater, hurling 10-12 ton boulders up to half a mile (800 meters) and scatter pebble-sized rocks over 12 miles (19 km), the USGS has said.

SEE ALSO: Kilauea Volcano: Code red warning issued as eruptions intensify

This type of eruption has the potential to carpet the Big Island in much thicker ash than current dustings and possibly spread the powder and volcanic smog across the Hawaiian islands and farther afield if it enters the stratosphere.

More on the Kilauea volcano:

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Volcano eruption in Hawaii forces evacuation
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Volcano eruption in Hawaii forces evacuation
PAHOA, HI - MAY 5: In this handout photo provided by the U.S. Geological Survey, lava from a fissure slowly advances to the northeast on Hookapu Street after the eruption of Hawaii's Kilauea volcano on May 5, 2018 in the Leilani Estates subdivision near Pahoa, Hawaii. The governor of Hawaii has declared a local state of emergency near the Mount Kilauea volcano after it erupted following a 5.0-magnitude earthquake, forcing the evacuation of nearly 1,700 residents. (Photo by U.S. Geological Survey via Getty Images)
PAHOA, HI - MAY 5: In this handout photo provided by the U.S. Geological Survey, lava errupts from a new fissure from Luana Street after the eruption of Hawaii's Kilauea volcano on May 5, 2018 in the Leilani Estates subdivision near Pahoa, Hawaii. The governor of Hawaii has declared a local state of emergency near the Mount Kilauea volcano after it erupted following a 5.0-magnitude earthquake, forcing the evacuation of nearly 1,700 residents. (Photo by U.S. Geological Survey via Getty Images)
PAHOA, HI - MAY 5: In this handout photo provided by the U.S. Geological Survey, a panoramic view of a fissure errupting lava from the intersection of Leilani and Makamae Streets after the eruption of Hawaii's Kilauea volcano on May 5, 2018 in the Leilani Estates subdivision near Pahoa, Hawaii. The governor of Hawaii has declared a local state of emergency near the Mount Kilauea volcano after it erupted following a 5.0-magnitude earthquake, forcing the evacuation of nearly 1,700 residents. (Photo by U.S. Geological Survey via Getty Images)
A man watches as lava is seen sewing from a fissure in the Leilani Estates subdivision near the town of Pahoa on Hawaii's Big Island on May 4, 2018 as up to 10,000 people were asked to leave their homes following the eruption of the Kilauea volcano that came after a series of recent earthquakes. (Photo by Frederic J. BROWN / AFP) (Photo credit should read FREDERIC J. BROWN/AFP/Getty Images)
A man watches as lava is seen coming from a fissure in Leilani Estates subdivision on Hawaii's Big Island on May 4, 2018. - Up to 10,000 people have been asked to leave their homes on Hawaii's Big Island following the eruption of the Kilauea volcano that came after a series of recent earthquakes. (Photo by FREDERIC J. BROWN / AFP) (Photo credit should read FREDERIC J. BROWN/AFP/Getty Images)
TOPSHOT - Lava is seen coming from a fissure in Leilani Estates subdivision on Hawaii's Big Island on May 4, 2018. - Up to 10,000 people have been asked to leave their homes on Hawaii's Big Island following the eruption of the Kilauea volcano that came after a series of recent earthquakes. (Photo by FREDERIC J. BROWN / AFP) (Photo credit should read FREDERIC J. BROWN/AFP/Getty Images)
TOPSHOT - Steam rises from a fissure on a road in Leilani Estates subdivision on Hawaii's Big Island on May 4, 2018. - Up to 10,000 people have been asked to leave their homes on Hawaii's Big Island following the eruption of the Kilauea volcano that came after a series of recent earthquakes. (Photo by FREDERIC J. BROWN / AFP) (Photo credit should read FREDERIC J. BROWN/AFP/Getty Images)
HAWAII VOLCANOES NATIONAL PARK, HI - MAY 3: In this handout photo provided by the U.S. Geological Survey, ash sprews from the Puu Oo crater on Hawaii's Kilauea volcano on May 3, 2018 in Hawaii Volcanoes National Park. The governor of Hawaii has declared a local state of emergency near the Mount Kilauea volcano after it erupted following a 5.0-magnitude earthquake, forcing the evacuation of nearly 1,700 residents. (Photo by U.S. Geological Survey via Getty Images)
HAWAII VOLCANOES NATIONAL PARK, HI - MAY 3: In this handout photo provided by the U.S. Geological Survey, a fissure forms on the west flank of the Puu Oo crater on Hawaii's Kilauea volcano on May 3, 2018 in Hawaii Volcanoes National Park. The governor of Hawaii has declared a local state of emergency near the Mount Kilauea volcano after it erupted following a 5.0-magnitude earthquake, forcing the evacuation of nearly 1,700 residents. (Photo by U.S. Geological Survey via Getty Images)
HAWAII VOLCANOES NATIONAL PARK, HI - MAY 3: In this handout photo provided by the U.S. Geological Survey, ash sprews from the Puu Oo crater on Hawaii's Kilauea volcano on May 3, 2018 in Hawaii Volcanoes National Park. The governor of Hawaii has declared a local state of emergency near the Mount Kilauea volcano after it erupted following a 5.0-magnitude earthquake, forcing the evacuation of nearly 1,700 residents. (Photo by U.S. Geological Survey via Getty Images)
HAWAII VOLCANOES NATIONAL PARK, HI - MAY 3: In this handout photo provided by the U.S. Geological Survey, the collapsed Puu Oo crater, which formed on April 30, spews ash on Hawaii's Kilauea volcano on May 3, 2018 in Hawaii Volcanoes National Park. The governor of Hawaii has declared a local state of emergency near the Mount Kilauea volcano after it erupted following a 5.0-magnitude earthquake, forcing the evacuation of nearly 1,700 residents. (Photo by U.S. Geological Survey via Getty Images)
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"This morning dense ballistic blocks up to 60 cm (2 feet) across were found in the parking lot a few hundred yards (meters) from Halemaumau (Kilauea's crater)," the USGS said in a statement. "These reflect the most energetic explosions yet observed and could reflect the onset of steam-driven explosive activity.

A 4.2 magnitude earthquake at the volcano at 8.36 a.m. (2:36 p.m. EDT) prompted authorities to issue an alert reassuring rattled Big Island residents that there was no risk of a tsunami from the volcanic activity.

'DISARRAY AND FRANTIC'

In the community of Volcano, just north of Kilauea's crater, business was way down and people were on edge.

In just a few hours residents had been shaken by an earthquake, dusted with ash and for the first time since the start of the eruption smelled the rotten-egg stench of toxic sulfur-dioxide gas.

"They're just in disarray and frantic," Adele Tripp, an employee at the Kilauea General Store, said of other residents. She said she had lived in Volcano for 30 years and was not personally concerned as she trusted scientists to tell her when to get out.

Smog from Kilauea drifted north up the island chain as the districts of Kau, Puna, and North and South Hilo were told by the National Weather Service to expect ashfall. Hawaii County Civil Defense said a dusting of ash was visible on property and advised residents to avoid exposure to the powdered rock, which can cause irritation to eyes and airways.

There was no effect on air carrier operations to Hawaii on Wednesday, Federal Aviation Administration spokesman Ian Gregor said in an email.

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Volcano erupts behind golfers in Hawaii
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Volcano erupts behind golfers in Hawaii
HAWAII VOLCANOES NATIONAL PARK, HI - MAY 15: People play golf as an ash plume rises in the distance from the Kilauea volcano on Hawaii's Big Island on May 15, 2018 in Hawaii Volcanoes National Park, Hawaii. The U.S. Geological Survey said a recent lowering of the lava lake at the volcano's Halemaumau crater 'has raised the potential for explosive eruptions' at the volcano. (Photo by Mario Tama/Getty Images)
HAWAII VOLCANOES NATIONAL PARK, HI - MAY 15: People watch at a golf course as an ash plume rises in the distance from the Kilauea volcano on Hawaii's Big Island on May 15, 2018 in Hawaii Volcanoes National Park, Hawaii. The U.S. Geological Survey said a recent lowering of the lava lake at the volcano's Halemaumau crater 'has raised the potential for explosive eruptions' at the volcano. (Photo by Mario Tama/Getty Images)
HAWAII VOLCANOES NATIONAL PARK, HI - MAY 15: A man drives a golf cart at a golf course as an ash plume rises in the distance from the Kilauea volcano on Hawaii's Big Island on May 15, 2018 in Hawaii Volcanoes National Park, Hawaii. The U.S. Geological Survey said a recent lowering of the lava lake at the volcano's Halemaumau crater 'has raised the potential for explosive eruptions' at the volcano. (Photo by Mario Tama/Getty Images)
HAWAII VOLCANOES NATIONAL PARK, HI - MAY 15: Kids play at a golf course as an ash plume rises in the distance from the Kilauea volcano on Hawaii's Big Island on May 15, 2018 in Hawaii Volcanoes National Park, Hawaii. The U.S. Geological Survey said a recent lowering of the lava lake at the volcano's Halemaumau crater 'has raised the potential for explosive eruptions' at the volcano. (Photo by Mario Tama/Getty Images)
HAWAII VOLCANOES NATIONAL PARK, HI - MAY 15: Boys play at a golf course as an ash plume rises in the distance from the Kilauea volcano on Hawaii's Big Island on May 15, 2018 in Hawaii Volcanoes National Park, Hawaii. The U.S. Geological Survey said a recent lowering of the lava lake at the volcano's Halemaumau crater has raised the potential for explosive eruptions at the volcano. (Photo by Mario Tama/Getty Images)
HAWAII VOLCANOES NATIONAL PARK, HI - MAY 15: A man drives a golf cart at a golf course as an ash plume rises in the distance from the Kilauea volcano on Hawaii's Big Island on May 15, 2018 in Hawaii Volcanoes National Park, Hawaii. The U.S. Geological Survey said a recent lowering of the lava lake at the volcano's Halemaumau crater �as raised the potential for explosive eruptions�at the volcano. (Photo by Mario Tama/Getty Images)
HAWAII VOLCANOES NATIONAL PARK, HI - MAY 15: People watch at a golf course as an ash plume rises in the distance from the Kilauea volcano on Hawaii's Big Island on May 15, 2018 in Hawaii Volcanoes National Park, Hawaii. The U.S. Geological Survey said a recent lowering of the lava lake at the volcano's Halemaumau crater 'has raised the potential for explosive eruptions' at the volcano. (Photo by Mario Tama/Getty Images)
HAWAII VOLCANOES NATIONAL PARK, HI - MAY 15: A woman hold her dog Tzippy at a golf course as an ash plume rises in the distance from the Kilauea volcano on Hawaii's Big Island on May 15, 2018 in Hawaii Volcanoes National Park, Hawaii. The U.S. Geological Survey said a recent lowering of the lava lake at the volcano's Halemaumau crater 'has raised the potential for explosive eruptions' at the volcano. (Photo by Mario Tama/Getty Images)
HAWAII VOLCANOES NATIONAL PARK, HI - MAY 15: A family gathers at a golf course as an ash plume rises in the distance from the Kilauea volcano on Hawaii's Big Island on May 15, 2018 in Hawaii Volcanoes National Park, Hawaii. The U.S. Geological Survey said a recent lowering of the lava lake at the volcano's Halemaumau crater 'has raised the potential for explosive eruptions' at the volcano. (Photo by Mario Tama/Getty Images)
VOLCANO, HI - Foreign tourists climb trees on the 18th hole of the Volcano Golf and Country Club to view the plumes of smoke coming from the Halemaumau Vent of the Kilauea Volcano in Volcano, HI on May 15, 2018. It is reportedly spewing only ash at this time and no large rocks. (Photo by Linda Davidson / For The Washington Post via Getty Images)
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Ash is a new hazard for Hawaii's Big Island, already grappling with volcanic gas and lava that has destroyed 37 homes and other structures and forced the evacuation of about 2,000 residents from a small area in the southeast Puna district.

Lava has burst from 21 giant ground cracks or fissures and torn through housing developments and farmland, threatening two highways that are exit routes for coastal areas.

Several fissures shot lava into the air on Wednesday but one flow advanced only 100 yards (meters) toward coastal Highway 137, which remains around a mile (1.6 km) distant, County of Hawaii Civil Defense said in a statement.

No serious injuries or deaths have been reported from the eruption.

(Reporting by Terray Sylvester; Additional reporting and writing by Andrew Hay in Taos, New Mexico; Editing by Tom Brown and Sandra Maler)

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